112 
MORPHOLOGY OF TISSUES. 
Monocotyledons these are still more conspicuous, especially if the bundle-sheath 
of lignified prosenchyma, which often occurs in them, is taken into account (see 
Fig. 92). Among Ferns, Lycopodiaceae (with isolated bundles^), and Rhizocarpeae, 
the xylem lies in the centre of the bundle, while the phloem forms a soft succulent 
sheath round it (Fig. 67, p. 80, and Fig. 94). 
According to the relative position of the two principal constituents of fibro- 
vascular bundles, they may be classified under two varieties: — (i) Collateral bundles, 
in which the phloem and xylem run parallel to one another, like the two halves of a 
Fig. 93.— Part of a transverse section from the, mature hypocotyledonary portion of the stem (tigellum) of Ricüins com- " 
mii>iis; r parenchyma of the primary cortex, rn of the pith, both belonjjing- to the fundamental tissue ; between r and b is the 
simple bundle-sheath containing' starch-grains, also belonging to the fundamental tissue. The fibro-vascular bundle consists 
of the phloem y, the xylem g-, t, and the cambium c c, and is therefore an open bundle. The cambium of the bundle 
c CK also continued into the fundamental tissue that lies between it and the neighbouring bundles in the form of interfascicular 
cambium cb, formed by subsequent divisions of large parenchymatous cells {secondary meristem). In the phloem are bast- 
fibres b b, the soft -bast J/ jK (partly parenchyma and partly sieve-tubes); in the xylem narrow pitted vessels it, broad pitted 
vessels and between them wood-fibres. 
rod bisected lengthwise, as occurs in Phanerogams and Equisetaceae ; and (2) 
Concentric, when the xylem is completely enclosed within a layer of phloem, as 
in Ferns, the simple bundles of Lycopodiaceae ^ &c. The latter kind are always 
^ The bundle in the stem of Lycopodium Chamcecyparissus, &c., is clearly a union of several 
fibro-vascular bundles, such as occurs in the axial cylinder of roots (see Book II., Lycopodiaceae). 
^ See Russow. Vergleichende Untersuchungen liber die I eitbündelkryptogamen. Petersburg 1872, 
p. 159 et seq. The terminology employed in this paper does not seem to me happy, with the 
exception of the term 'collateral bundles.' 
